The Sun Is Gradually Increasing in Luminosity as It Ages

The Sun is gradually increasing in luminosity as it ages. It is 20% more luminous than it was when it was born. Why isn't this considered to be an explanation for global warming?

A) The Sun is increasing in luminosity much too slowly, over many millions of years.
B) The scientists are covering up this potentially valid explanation.
C) The amount of energy the Earth receives from the Sun remains constant regardless of what the Sun does.
D) What the Sun was doing millions of years ago is what determines Earth's temperature, since the Earth doesn't respond directly to what the Sun is doing today.